A German nurse, Niels Hoegel, is currently on trial facing charges of killing 100 patients.
It is his third trial, and he is serving a life sentence for murdering two patients and playing a role in the killing of four others.
Today, Hoegel, 42, is considered the most prolific serial killer in the history of peacetime Germany, and perhaps in the world. Officials suspect that as many as 300 patients may have died by his hand over five years starting in 2000.
Prosecutors said he created situations in which life and death rested in his own hands.
He administered overdoses of drugs that caused cardiac arrest so that he could rush back and try to revive patients heroically. His colleagues called him "Resuscitation Rambo" and rewarded his skill with a necklace made of injection tubes, which he wore with pride.
Here are some other notorious serial killers.
1. Ted Bundy (US)
The American serial killer and rapist is one of the most notorious in history. Ted Bundy was involved in the disappearance of more than 30 young women and girls in the United States in the 1970s.
He tended to prey on young and attractive college women, first near his home in Washington, then moving east to Utah, Colorado and Florida.
He would trick the women by wearing his arm in a sling or putting his leg in a cast, and convince them help him carry books or unload objects from his car.
He was also known to impersonate authority figures.
Once they got to his 1968 tan Volkswagen Beetle, he would strike them over the head and immobilise them with handcuffs.
He typically strangled or bludgeoned his victims as well as mutilated them after death, and also was known to return to visit the corpses at their dump sites or take them home for further sexual gratification.
During his court trials, his charm and intelligence, as well as the appalling nature of his crimes, drew significant public attention, and his escapes from prison - twice - furthered his notoriety.
He was put to death via electric chair in 1989.
His case has become the basis of several books and films, including the recently released crime drama Extremely Wicked, Shockingly Evil And Vile, with Zac Efron taking on the lead role.
2. Jack the Ripper (UK)
A notorious serial killer who terrorised the streets of London more than a century ago, Jack the Ripper's real identity was a mystery for many years.
He is believed to have killed at least five women in the Whitechapel area of London in 1888. No one was ever charged in the murders.
All five women were prostitutes, and four of them were horribly mutilated.
Some of them had their throats slit while others had organs removed.
Jack the Ripper has appeared as a character in, and inspired books, theatre, films and even music.
In March this year, genetic tests published in the Journal of Forensic Sciences pointed to Aaron Kosminski, a 23-year-old Polish barber and a prime police suspect at the time, according to CBS News. Kosminski died in an asylum in 1919.
3. Yang Xinhai (China)
The introverted drifter cycled from town to town in China, killing entire families. Yang Xinhai murdered most of his victims in their sleep, using axes and shovels, and often raped the women.
The murders began in 2000, and were carried out over three years.
The 35-year-old was arrested in 2003 for killing 65 people and executed a year later.He was a convicted rapist and robber whose time in prison and labour camps had reportedly turned him against society.
In one case, Yang killed a father and a six-year-old girl with a shovel and raped a pregnant woman, who survived the attack with serious head injuries.
His parents told Beijing News then that they had not heard from him for years after he dropped out of school and started drifting around the country doing odd jobs.
4. Harold Shipman (UK)
Known as Dr. Death, British family physician Harold Shipman was believed to have killed as many as 250 people over more than 20 years, most of them elderly and middle-aged women who were his patients.
Most of them were supposedly killed with lethal drug injections.
Shipman was convicted in 2000 and sentenced to 15 life terms.
Police investigations revealed his modus operandi - to visit the home of elderly patient, give her an injection, pop back to his surgery to tinker with his records and then return to pronounce his victim dead, often with a callous mutter: "She's gone."
He was eventually found out when he tried to profit from a murder by forging a woman's signature on a will he drafted, naming himself as the sole beneficiary, then killing her.
The woman's solicitor daughter then pursued the death with the police.
He died in prison in 2004, aged 57, in an apparent suicide.
5. Bruce McArthur (Canada)
Landscaper Bruce McArthur was sentenced to life in prison in February this year, ending a trial that shocked a city - and a country - that likes to see itself as inclusive and safe.
For years, members of Toronto's gay community warned that there was a serial killer on the loose, that vulnerable men were going missing, that the streets were not safe. They were right.
McArthur was accused of killing and dismembering eight men between 2010 and 2017, hiding seven of the corpses in planters and the eighth in a ravine, reported Washington Post.
At a sentencing hearing in February, Canadians heard how he lured and murdered men he met in Toronto's Gay Village, then posed corpses in costumes, keeping pictures of each victim in labelled digital folders.
They learned McArthur was stopped when police raided his home, finding a man tied to a bed. He was a potential ninth victim, the court heard, and McArthur had a folder waiting.
In a city that prides itself on being gay-friendly and welcoming to new Canadians, McArthur sought out men marginalised by their sexuality, ethnicity, immigration status or poverty. Most of his victims were refugees or immigrants. Several struggled with substance abuse. Some had not revealed they were gay.
6. Tsutomu Miyazaki (Japan)
In a 1988 to 1989 serial killing spree that shocked the nation, Tsutomu Miyazaki abducted and murdered four girls, aged between four and seven years old.
His first victim, a four-year-old, was kidnapped in the Saitama prefecture and strangled in a forest in Tokyo.
Five months later, he burned the body and left the ashes in a cardboard box in front of the victim's home, the Japan Times reported in 2001, when the Tokyo High Court upheld the death penalty for him.
The other three victims were also strangled, and for one of the victims, Miyazaki testified in district court that he ate her wrists.
Miyazaki mutilated the bodies of the victims, slept next to the corpses and drank their blood, according to a Japan Today report in 2008 when the killer was executed.
In a court session, Miyazaki admitted killing the four girls, but said he committed the crimes "as if in his permanent dream" and claimed he lost his mind after "rat people" appeared in his vision prior to the killings.
Miyazaki was hanged in 2008 at the age of 45.
SOURCES: National Museum of Crime and Punishment, Encyclopaedia Britannica, CBS News, Reuters, Japan Times, Japan Today, Washington Post, AFP, South China Morning Post